An ultrasonic vibrator horn used on the ultrasonic atomizing apparatus of the type described is typically connected to an electroacoustic transducer which, together with a high frequency oscillator forms ultrasonic vibration generating means. High frequency electric oscillations produced by the high frequency oscillator as it is driven are applied to the electroacoustic transducer which converts the electric vibrations into ultrasonic vibrations which are used to atomize liquid feed. With the ultrasonic vibrator horn of the type described above, it is known that the atomization properties such as the flow rate of spray of liquid material such as liquid fuel as it is atomized upon being fed from a liquid feeding mechanism in the ultrasonic atomizer and the particle size of the atomized droplets will have various effects on the performances of combustors such as boilers, gasoline engines and the like in which the ultrasonic atomizer is used. By way of example, poor atomizing properties of the ultrasonic vibrator horn can make it impossible to effect positive control of the ratio of air and fuel, or deteriorate the combusting conditions to result in an increase in the content of hydrocarbon and carbon monoxide in the exhaust gases as well as an increase in soots produced.
In order to eliminate such problems with the combustion apparatus it is required to improve the atomizing characteristics of the ultrasonic vibrator horn as described above.
To this end, various improvements on the ultrasonic vibrator horn have been heretofore proposed as illustrated in FIGS. 6A-D and 7A-C.
The prior art vibrator horn 109 shown in FIG. 6A is characterized by a plurality of flutes or longitudinal grooves 107 formed in the peripheral surface of the horn for receiving and directing liquid fuel from the feed point of liquid fuel, that is, from the liquid feed mechanism or fuel feed pipe 103 positioned adjacent the periphery of the horn towards the forward end of the horn. The grooves 107 are either V-shaped in cross-section as shown in FIG. 6B, or U-shaped as shown in FIG. 6C, or channel-shape as shown in FIG. 6D. The grooves may extend parallel to the central axis of the horn or may alternatively be inclined progressively towards the central axis as it proceeds towards the forward end of the vibrator horn 109, as required. The grooves 107 formed in the outer periphery of the vibrator horn 109 are designed to direct the liquid fuel fed to the vibrator horn towards the forward end of the horn which defines an atomizing area, to prevent the liquid fuel from dropping off the horn before reaching the forward end thereof even when the horn is disposed in a horizontal orientation, and to provide an increased surface area and an enhanced cooling effect.
The proposal as illustrated in FIGS. 7A-7C comprises one or more longitudinal grooves 117 formed in the peripheral surface of the ultrasonic vibrator horn 119 of the type which has equal-diameter multiple-stepped edged portions extending from a point intermediate the ends of the horn to the forward end. The groove or grooves 117 are either V-shaped, or U-shaped, or channel-shaped in cross-section and extend from the feed point of liquid fuel discharged from a feed pipe 113 to the edged portion one step short of the foremost edged portion for receiving and directing the liquid fuel. The grooves 117 may extend parallel to the central axis of the horn 119 or may be inclined progressively towards the central axis as it proceeds towards the forward end of the vibrator horn, as required. The grooves 117 formed in the outer periphery of the vibrator horn 119 are designed to direct the liquid fuel fed to the horn towards the equal-diameter multiple-stepped edged portions which define the atomizing area of the horn, to prevent the liquid fuel from dropping off the horn before reaching the edged portions even when the horn is disposed in a horizontal orientation, to ensure good distribution of fuel over the edged portions and hence stable atomization of fuel, and to provide an increased surface area and an enhanced cooling effect. The foremost edged step devoid of grooves serves as a stop to keep the fuel flowing along the grooves 117 from flying off or flowing out of the atomizing area.
With the ultrasonic vibrator horn 109 according to the proposal as illustrated in FIG. 6A, however, the atomizing area for atomizing the liquid fuel as supplied from the feed pipe 103 is limited to the forward end portion of the vibrator horn 109, so that when the horn is disposed in a horizontal position as shown in FIGS. 8A and 8B, the spatial extent F of spread of the atomized droplets is restricted to a narrow strip of space extending generally straight forward from the horn 109, resulting in a relatively low rate of spray or atomization and disadvantageously large particle size of atomized droplets. With the ultrasonic vibrator horn 119 according to the proposal as illustrated in FIGS. 7A-7C, the atomizing area for atomizing the liquid fuel as supplied from the fuel feed pipe 113 extends over all the edged portions except the foremost one so that the atomizing area is considerably larger than that of the vibrator horn 109 shown in FIGS. 8A and 8B. Consequently, when the horn 119 is oriented in a horizontal position as shown in FIGS. 9A and 9B, it provides an adequately wide extent F of spread of atomized droplets. However, if the feed point of liquid fuel from the feed pipe 113 to the vibrator horn 119 is changed, or if the flow rate of fuel supply varies, then the spatial extent of spray spread will vary, so that the spray spread extent F is unstable.
As a result of extensive researches and experiments conducted to overcome the aforesaid problems with the prior art, the present inventors have found out that in an ultrasonic vibrator horn having groove means formed in the outer periphery of the horn, said groove means communicating with the axially forward end portion and adapted to receive and direct liquid fuel as fed thereto toward said forward end portion, it is advantageous to make the end portion of the horn conical in shape so that the atomization of the liquid fuel takes place at the conical forward end portion and in its vicinity, and that the spacial extent of spray spread of the liquid fuel extends over an angle of 180.degree. minus the vertical angle .alpha. of the conical forward end portion of the vibrator horn. The inventors have thus discovered that it is possible to define an optimal spatial extent of spray spread depending on the size and configuration of the combusting apparatus, for example, with which the vibrator horn is used by setting the vertical angle of the conical forward end portion of the horn at an appropriate value.
The present invention is based on such novel discovery.